In not only an image reading apparatus, but also an electronic computer and peripheral devices thereof (refer to the electronic computer and the peripheral devices thereof to as a “computer or the like”, hereinafter), the generation of noise due to the influence of an electromagnetic wave needs to be reduced. The computer or the like is requested to perform various kinds of processes at high speed. In order to perform the processes at high speed in the computer or the like, the frequency of a clock signal for adjusting the operating timing of elements or circuits respectively forming parts of the computer or the like. At present, the frequency of the clock signal is widely set to several MHz to several GHz.
However, it has been known that, when the frequency of the clock signal is raised, that is, when a high frequency is used for the clock signal, the generation of an electromagnetic shield noise, what is called an EMI (Electro Magnetic Interference) noise increases. The EMI noise has a problem that the EMI noise is amplified when a switching operation (for instance, 0 to 1 or 1 to 0) is performed at the same time synchronously with the clock signal, for instance, upon outputting the signal.
To reduce the EMI noise, a special interface standard such as an ECL or an SSTL has been hitherto used, cables of a transmission system or the like have been coated with shield members, or EMI filters have been provided in all signal lines to reduce a noise level.
However, in the above-described case, the special interface standard is used or the shield members or the EMI filters need to be added. Consequently, a cost is increased.
Further, even when the special interface standard is used or the shield members or the EMI filters are added as described above, the amplification of noise due to the simultaneous switching synchronous with the clock signal is hardly reduced.
On the other hand, an image reading apparatus which has an image pick-up unit such as a CCD (Charge Coupled Device) to form digital image data from an analog electric signal outputted from the image pick-up unit and output the digital image data has been known. From the image pick-up unit of the image reading apparatus, the analog electric signal is outputted as described above. The analog electric signal is outputted at a prescribed timing for each line. Therefore, the analog electric signal outputted from the image pick-up unit needs to be specified to a prescribed output timing. Not only the image reading apparatus, but also the computer or the peripheral devices thereof use the clock signal of a prescribed frequency to specify the input and output timing of data or a calculating timing. The data is inputted and outputted or calculated synchronously with the clock signal.
In this image reading apparatus, a control signal synchronous with the clock signal is used to specify the output timing of the analog electric signal from the image pick-up unit.
In a usual image reading apparatus, for instance, two control signals are supplied to image pick-up unit to specify the output timing of an analog electric signal in accord with a cross-point at which the two control signals intersect. That is, the analog electric signal is outputted from the image pick-up unit at the cross-point where the two control signals intersect.
To allow the image pick-up unit to recognize the cross-point, voltage at the cross-point needs to be located within a prescribed range.
However, the voltage at the cross-point is different for each type of the image pick-up unit, that is, every time the kind of the image pick-up unit or a substrate changes. Each type of image pick-up unit has specific voltage characteristic of each image pick-up unit. Therefore, the voltage at the cross-point of the control signals needs to be set to the specific voltage for each of the image pick-up unit. When the voltage at the cross-point is not the specific voltage, the analog electric signal is not outputted from the image pick-up unit. Accordingly, each image pick-up unit needs to be strictly adjusted so that the voltage at the cross-point becomes the specific voltage.
Thus, a resistance element or a capacitor element or the like has been hitherto disposed in a control signal circuit part for supplying a control signal to image pick-up unit. Thus, a resistance value or a capacity thereof has been adjusted to adjust the waveform of the control signal and voltage at a cross-point.
However, since the specific voltage is different for each image pick-up unit, the resistance value or the capacity of the resistance element or the capacitor element needs to be adjusted for each image pick-up unit. Therefore, after the image pick-up unit is disposed on, for instance, a substrate, an externally attached adjusting circuit including a resistance element or a capacitor element needs to be provided to adjust the waveform of a control signal in accordance with the specific voltage of the image pick-up unit. As a result, the adjustment of the voltage at the cross-point disadvantageously becomes complicated.
Accordingly, it is an object of the present invention to provide an image reading apparatus in which EMI noise is reduced and the amplification of noise due to a simultaneous switching operation is reduced without requiring the use of a special interface standard or the addition of a shield member.
Further, it is another object of the present invention to provide an image reading apparatus in which voltage at a cross-point is easily adjusted without requiring the adjustment of a waveform and an externally attached circuit.